Carbon fibers have small ductility and are relatively brittle, so they easily become fuzzy as a result of mechanical friction. In order to improve their handling properties, carbon fibers are usually sized. As sizing agents for use with carbon fibers, polyglycidyl ether (described in Japanese Patent Publication No. 15229/82) and various other compounds have been proposed.
Thus, the primary function of the sizing agent is to prevent any mechanical damage to the tow, thereby maintaining the strength properties of the carbon fibers, and improving its handling. Our extensive studies on carbon fibers have revealed that carbon fibers having uneven and striated surfaces are disintegrated fairly easily and cannot be bundled to form a tow having high alignment. In order to prevent the formation of excessive fuzzs and to improve the handling of the tow, such carbon fibers must be sized to have a surface coat of a viscous oligomer having a relatively high molecular weight. On the other hand, carbon fibers having smooth surfaces can be easily bundled into a tow having good handling properties. Therefore, such carbon fibers need not be sized but may only have to be wetted with water for proper operation of the subsequent steps. If a viscous oligomer coat is formed on such smooth surfaced carbon fibers, the individual fibers are bundled so tightly as to reduce the spreadability of the resulting tows. This will cause various problems in subsequent operations, such as insufficient impregnation of the fibers with the matrix resin in the prepregging step. Another characteristic feature of the smooth surfaced carbon fibers is that the slightest change in the deposition of the size will cause great variations in the bundling property of the fibers and the spreadability of the tows.
It is therefore important that sizing agents of different characteristics should be used depending upon the surface properties of the carbon fibers to be treated. From an industrial viewpoint, it is desired to develop a size that has a broad working range and which will not cause great variations in the bundling property or tow spreadability even if there occurs a certain change in the amount of the size being deposited.
The secondary function of the size is to control the characteristics of interface between the carbon fibers and the matrix resin in a composite. A common matrix resin is an epoxy resin but if a silicone resin oligomer is used as a size, for instance, the strength of the interface between the resin and the carbon fibers is remarkably decreased and the desired composite is not obtainable. This is probably because the silicone resin has poor solubility with the epoxy resin. In most practical applications, epoxy resin oligomers are used as the sizing agent and it is generally understood that good interface strength is ensured since the size at the interface between the matrix epoxy resin and the carbon fibers is simultaneously cured by the curing agent in the matrix resin. As shown in J. Adhesion, 16, 133, 1983 and the Proc. of the 38th Ann. Conf. of SPI. Sec. 12-F, 1983, the composite forming properties of the carbon fibers could be improved by making use of the ability of the size to increase the adhesion strength at the interface between the fibers and the matrix resin. However, it is also important to maintain the good spreadability of individual tows of carbon fibers and considerable difficulties exist in obtaining good balance between the bundling and spreading properties of the fibers. In fact, most of the conventional sizing agents have been developed with a view to maximizing the bundling property of the carbon fibers without impairing their composite properties.
The conventional sizing agents are classified into two types: the solution type using an organic solvent and the emulsion type having the size dispersed in water with the aid of an emulsifier. One major defect with the size of the solution type is that the organic solvent which can cause void formation must be completely removed with heat during the fabrication of a composite. For this reason, the solvents that can be used are restricted to those which boil at low temperatures, such as methyl ethyl ketone and alcohol. Additionally, the prerequisite for the use of organic solvents is the provision of special facilities that will ensure maximum safety and protection against both health hazards and environmental pollution. Furthermore, only limited types of sizing agents are dissolvable in the usable solvents. These problems are absent from the sizing agent of the emulsion type, as was already proposed by the present inventors in Japanese Patent Application (OPI) No. 171767/82 (the term "OPI" as used herein refers to a "published unexamined Japanese patent application open to public inspection"). However, most of the epoxy resin oligomers are not easily emulsifiable and because of their high degree of instability, scum easily forms in the emulsions. An emulsion of the epoxy resin oligomer could be formed by using a great amount of emulsifier, but this is detrimental to the purpose of providing good composite properties and a sizing agent with a high content of the emulsifier cannot be uniformly deposited on the carbon fibers. With all these factors taken into consideration, a water-soluble size would be the best material. On the basis of this recognition, the present inventors conducted studies on the water-soluble epoxy resins, and proposed the use of a polyol polyglycidyl ether in Japanese Patent Application (OPI) No. 128266/82.
However, later studies have revealed that a composite made from carbon fibers sized with this polyol polyglycidyl ether is poor in its hot wet properties. A probable reason for this phenomenon would be that polyol polyglycidyl ethers have such low molecular weights that they are extremely soluble in water.